In cellular mobile communication systems, a radio coverage area is implemented with a plurality of slightly overlapping radio cells. When a mobile station moves from one cell to another, handover to a new radio cell is performed on the basis of predetermined handover criteria. The aim is to perform handover in a manner that disturbs an ongoing call as little as possible. Handover is normally performed on the basis of radio path criteria, but it may also be performed for other reasons, e.g. to divide the load, or to reduce transmission powers, or when a mobile station has moved too far from the base station, in which case the propagation delay of the radio signal becomes too great.
FIG. 1 of the appended drawings shows a simplified block diagram of the pan-European GSM mobile communication system. A mobile station MS is connected over a radio path to a base transceiver station BTS, in the case of FIG. 1 to BTS1. A base station system BSS consists of a base station controller BSC and base stations BTS controlled by the BSC. A plurality of base station controllers BSC usually operate under the control of a mobile services switching centre MSC. An MSC communicates with other MSCs and, through a gateway mobile services switching centre GMSC, with a public switched telephone network. The operation of the entire system is controlled by an operation and maintenance centre OMC. The subscriber data of a mobile station MS are stored permanently in a home location register HLR of the system and temporarily in the visitor location register VLR in the area of which the MS is located at a given moment.
A mobile station MS and the serving base station BTS1 continuously measure the signal level and quality of the radio connection for example to determine the need for handover. The MS measures the signals of the serving base station BTS1 and the base stations BTS that are closest to its location area for instance to select a suitable target cell for handover. In the GSM mobile communication network, for example, an MS may simultaneously measure the signal level of both the serving base station and up to 32 other base stations. Via the serving base station BTS1, the MS is informed of the neighbouring cells it should measure. The measurement results of each cell are identified on the basis of the combination of a base station identity code BSIC and the frequency of the broadcast control channel BCCH.
The mobile station MS sends the measuring results regularly as a report message through the serving base station BTS1 to the base station controller BSC. A report message contains the measurement results of the serving base station and up to six best neighbouring base stations. Handover from a serving cell to a neighbouring cell may take place, for example, when the measurement results of the mobile station/base station indicate a low signal level and/or quality and a higher signal level is achieved in the neighbouring cell, or when a neighbouring cell allows communication at lower transmission powers. In addition, a handover to the most suitable neighbouring cell is performed when the serving base station is overloaded, or when an MS has moved too far from the serving base station BTS, or if it is necessary to change the base station for some other reason. The selection of the target cell for handover is influenced, for example, by the signal level and/or load of the target cell. In order for the stability of the mobile communication network to be ensured, the measurement results and parameters used in handover are averaged over a given period of time. The averaging process makes handover less sensitive to measurement results distorted by instantaneous interference or fading.
The base station controller BSC makes the decisions relating to handover. If the target cell is controlled by another BSC, the handover may be performed under the control of the MSC. Another possibility is that handover decisions are always made in a centralized manner in the MSC. If necessary, the BSC gives a handover command to the MS through the BTS.
In a mobile communication system implemented by code division multiple access (CDMA) technology, handover performed in the manner described above is called hard handover. In addition, CDMA systems can use so-called soft handover, in which a mobile station, during a handover, may be simultaneously connected to the network through several base stations. When one of these base stations proves to be better than the others on the basis of its signal, the connections of the mobile station with the other base stations are released, and the call is continued only through the best base station. Soft handover prevents recurring handovers between base stations when a mobile station is located on the periphery of cells.
Transmission errors which deteriorate the quality of a transmitted signal occur on the transmission path when speech or data is transmitted in a digital telecommunication system. Transmission errors occur on the transmission path when a signal is disturbed, for example, on account of multipath propagation, an interfering signal or high background noise level. Error correction of a digital signal to be transmitted, e.g. channel coding and/or retransmission, is used for improving the quality of the transmission and the tolerance of transmission errors. In channel coding, repetition is added to the original bit string of encoded speech or data by error correcting bits calculated from the original signal. In the receiver, the channel coding is decoded in a channel decoder, whereby the signal errors that have occurred during the transmission can be detected or even corrected by means of the correcting bits. Retransmission is used for correcting transmission errors either independently or, for example, in addition to channel coding: the errors in a channel coded transmission are corrected by retransmission of distorted frames. When the quality of the connection deteriorates, the number of erroneous and lost frames grows, and thus also the number of retransmissions grows.
Channel coding increases the number of bits to be transmitted. In the GSM mobile communication system, for example, error correcting bits with a transmission rate of 9.8 kbit/s are added to a full-rate 13 kbit/s speech signal, whereby the total transmission rate is 22.8 kbit/s. The level of the protection provided by channel coding is arranged according to the need. If a large number of data are to be transmitted fast, the amount of channel coding is reduced to allow more payload to be transmitted on the transmission channel. Channel coding may be arranged either to both detect errors occurred during the transmission and correct them or to merely detect them. In the GSM system, bits to be transmitted are divided according to their importance into different classes, in which channel coding is provided at a predetermined level. The different elements of the mobile communication system may limit the selection and implementation of the channel coding provided for a connection. A mobile station may, for instance, support only certain channel codings. Furthermore, the protection level of the provided channel coding depends on the capability of the base station and other network elements to employ different channel codings.
A problem with the handovers of the prior art is that the quality of the radio connection after handover is not always sufficient for a connection using weak channel coding. Another problem is that since network planning is usually intended for channels using normal channel coding, a handover is not performed to a target cell whose signal level and/or quality is too low for a connection using normal channel coding but sufficient for a connection employing more efficient channel coding.